Mix together the sugar syrup. Beat in the inverted sugar.
Blend the sugar syrup, the sparkling wine, the liqueur, and the remaining lemon juice.
If you are cautious, check that the sugar level is appropriate (the Egg test should give a good enough indication).
Churn in your ice cream maker according to instructions (avoid overchurning!)
[if using a raw egg white, whip it lightly-stiff and add to the sorbet base towards the end of the churning. If using Italian meringue or Aquafaba, also add these towards the end of the churning].
Consume on the spot or put in freezer-safe container, cover with plastic film and lid, and put in the freezer.
As this sorbet does not store well (especially if not "boosted" with egg white/Italian meringue/Aquafaba), it should preferably be enjoyed soon after preparation and not saved for a later day!